I wondered the same thing myself.

I tried the install without the included SPEED sensor (the one that came with the computer) and the computer would not pair with the Speedtrap sensor in the fork. I got an ERROR message.

When I tried it with the included SPEED sensor, the unit paired no problem.

I don't think it is supposed to work like that though. The whole purpose of the Speedtrap integration is to not have the SPEED sensor mounted to the bike, right?